(a) Field of the Invention
The present invention relates to an improved plasma reactor for use to treat ores or other metallic or non metallic compounds at a very high temperature in order to physically or chemically transform the same.
(b) Description of the Relevant Art
Plasma reactors are well known devices that have been made the subject of numerous research and development over the last decades. By definition, such reactors make use of a plasma, forming a heat generating arc column between a set of electrodes to heat the ores or compounds to be treated at very high temperatures and thus allow reactions to occur that would not be obtainable otherwise. The plasma forming the arc column consists of a mixture of energized and/or dissociated molecules, positively charged ions and free electrons obtained from a gas (hereinafted called "plasma gas") subjected to partial ionization by means of an electric arc (usually d.c.) formed between an anode and a cathode.
In practice, the plasma gas may often be used as a reactant. Thus, by way of example, oxygen or air can be used for carrying out oxidation. Carbon monoxyde or hydrogen can be used for carrying out reduction. Chlorine can be used for carrying out chlorination and nitrogen for nitration.
The plasma reactors that are presently available make use of devices called "plasma torches" to initiate and maintain the plasma in the reactor. These plasma torches may be of two different designs.
According to one of these designs called "hollow torch" the plasma torch comprises two tube-shaped electrodes made of copper or stainless steel, that are coaxially aligned and whose ends are kept apart by means of a small insulating ring. The plasma gas is injected between the electrodes through a hole made in the insulating ring. The plasma generated by the arc formed between the electrodes emerges from the torch whose electrodes act as a nozzle, and forms a jet of very high velocity.
It is of conventional practice to inject the gas tangentially in such a hollow torch to create a vortex and thus stabilize the arc along the axis of the nozzle. It is also conventional to use in some cases magnetic coils around one or both electrodes. This forces the points of attachment of the arc to the electrodes to rotate thereby reducing the risk of erosion over the electrode surface. It is further conventional to cool the electrodes with water jackets in order to increase their life expectancy.
According to the other design hereinafter called "solid torch", the plasma torch comprises a solid cathode usually in the form of a tip of tungsten, which is centrally held by an insulator inside a hollow anode. The plasma gas is injected in the annular space between the tungsten tip and its surrounding anode.
Once again, it is of conventional practice to incorporate cooling means to such a solid torch in order to increase the life expectancy of its components, and more particularly of its tungsten tips.
Whatever be the design of the plasma torch being used in the reactor may be, there are two basic approaches to using this torch and to operating the reactor.
One of these approaches consists of using the torch to generate a plasma flame, which is projected onto the material to be treated, in order to heat the same at the requested treatment temperature. In this approach, the plasma arc column permanently extends between the "built-in" anode and cathode of the torch, even if this column may be blown out of the same at a given distance by the injected plasma gas, and form an elongated loop.
The other approach consists of using the plasma torch and more particularly, the two built-in electrodes of the torch to generate a plasma arc column therebetween just at the beginning of the process, and subsequently transferring the so generated arc column to the material being treated which is usually in a molten form in the bottom of the reactor. Such an arc transfer is achieved in practice by switching one of the power source connections from the downstream electrode in the case of a hollow torch, or from the sleeve-shaped anode in the case of a solid torch, to another electrode (usually an anode) in contact with the bath of molten material at the bottom of the reactor.
Of course, this arc-transfer approach can only be used if the molten material being treated is electrically conductive. However, when this approach is used, it has several advantages:
it avoids anode heat loss; PA1 it distributes the arc power more evenly inside the reactor; and PA1 it keeps the bath hot at the bottom of the reactor (although it may overheat the point thereof where the arc strikes down). PA1 a vertical electrically insulated sleeve having an upper end, a lower end and an internal wall cylindrical in shape; PA1 a hollow torch coaxially mounted at the upper end of the sleeve, the torch comprising at least one tube-shaped electrode coaxial with the sleeve for use to generate a plasma arc column; PA1 means for injecting a gas tangentially into the hollow torch in order to create a vortex inside the same and thus stabilize the plasma arc column extending from the tube-shaped electrode.; PA1 means for dropping the powder material to be treated vertically downwardly inside the sleeve from the upper end thereof beside the hollow torch, so as to form a substantially uniform cylindrical curtain of particles falling down into the sleeve, these particles being centrifugally projected against the internal wall of the sleeve by the vortex escaping from the hollow torch and entirely covering the internal wall to shield the same while they are being simultaneously treated by the plasma column; PA1 a crucible positioned under the sleeve to collect the treated particles in molten form that drip down from the sleeve at the lower end thereof; and PA1 another electrode cooperating with the tube-shaped electrode of the hollow torch to generate the plasma arc column by proper connection of both of these electrodes to an electric power source. PA1 (a) It eliminates the use of a tungsten-tip electrode and its need for special arc gas and for protection from oxidation. PA1 (b) The process may use any gas. By way of example, CO may be use as a good reducing plasma gas compatible with the copper electrode of the standard hollow torch. PA1 (c) The hollow torch uses more plasma gas than the tungsten torch. The higher gas flow rate generates a strong tangential force which will assist the material feed injection. As a result, variable-size feed material may be dropped into the feed pipe. PA1 (d) The hollow torch is known to generate a stable plasma arc column. This permits to use of a tall sleeve pipe, for a long residence time for melting/smelting. PA1 (e) Restarting the hollow torch in case of arc outage is quite simple simple as compared to a solid torch where one always worries of tungsten-tip oxidation. PA1 (f) The electrodes may be of any polarity needed for the process.
U.S. Pat. No. 3,856,918 assigned to AMERICAN CYANAMID COMPANY discloses a non-transferred arc plasma reactor comprising a solid or hollow torch axially mounted on top of a vertical sleeve. The material to be treated is injected pneumatically at an angle on top of the sleeve, just under the torch, so as to impinge on the internal wall of the sleeve and flow down the same into a crucible. Meanwhile, it is heated by the plasma flame blown by the torch into the sleeve.
U.S. Pat. No. 4,002,466 assigned to BETLEHEM STEEL CORPORATION discloses a transferred arc plasma reactor comprising a solid torch axially mounted on top of a vertical sleeve or "collimator". The arc is transferred from the cathodic tip of the torch to the internal wall of the collimator which is connected to the power source and acts as an anode. The material to be treated is pneumatically injected tangentially near the top of the collimator and forms a falling film on the internal surface of the collimator. The arc strikes this film randomly while it flows down, and thus heats the material to the desired treatment temperature, before it falls into a crucible. The main advantage of this reactor is that the falling film protects the internal surface of the collimator acting as the anode and thus decreases the rate of erosion of the same. It also acts as a thermal insulator and thus decreases the heat loss at the anode which is externally cooled.
U.S. Pat. Nos. 3,932,171 and 4,154,972 both assigned to TETRONICS RESEARCH AND DEVELOPMENT COMPANY LTD. disclose a transferred arc plasma reactor in which the arc column is transferred from the cathode of a hollow torch to a bath of molten material in contact with an annular anode. The hollow torch is mounted at an angle on top of the reactor and is made to orbit about a vertical axis coaxial to the anode in order to permanently aim at the same. The material to be treated is merely dropped downwardly into the reactor through a plurality of openings surrounding the torch on top of the reactor. This forms a substantially uniform cylindrical falling curtain which is "swept" and heated by the plasma arc column while the torch is rotating.
Last of all, U.S. Pat. No. 4,466,824 originally granted to NORANDA MINES LTD. and subsequently assigned to HYDRO QUEBEC, discloses a transferred arc plasma reactor of substantially the same design as the one disclosed in the above mentioned U.S. Pat. No. 4,002,466, except that the anode is located at the bottom of the crucible, where the molten material flowing from the sleeve or collimator drops down. Of course, the collimator, in which the material to be treated is tangentially injected, is electrically insulated from the anodic bath, so that the arc extends through the sleeve between the cathodic tip of the torch and the bath formed by the drops of material molten along the sleeve by the heat generated by the plasma arc column. In this embodiment, the solid torch is also vertically movable to bring the cathodic tungsten tip close to the bath in order to start the process. The material in powder form is also projected by a carrier gas against the internal surface of the sleeve to form the requested falling film, which advantageously acts as a shield and thus limits wear of the sleeve surface, and heat loss through the same.
The main advantage of the design in U.S. Pat. No. 4,466,824 over the other known designs, including the one in U.S. Pat. No. 4,002,466 is essentially that it increases the thermal efficiency of the process. However, the design in U.S. Pat. No. 4,466,824 has the same major disadvantage as the other known designs of the same type, namely, the absolute necessity of using a carrier gas under substantial pressure to carry the pulverulent material to be treated through feed pipes leading from a storage hopper to the reactor and then injecting this material tangentially with a sufficient velocity to create a uniform cylindrical film covering the whole internal surface of the sleeve.